An example of a coupling of this type is shown in British patent 1 520 742. The coupling described in this patent includes metal teeth which grip into the smooth surface of a pipe and normally resist pull-out forces tending to separate the pipes. However in this coupling the metal teeth damage the pipes surface on assembly and this can lead to a reduction in sealing performance.
Another example of this type of coupling is shown in U.S. Pat. No. 4,191,408. In this specification, a shaped tube end is pushed into a socket where it is latched in place by a snap-fit action and engages with a seal ring to form a seal. However in this case, the tube end makes a seal with the seal ring before the snap action takes place to latch the tube in the socket. As a result, it is possible for the seal to be only partly assembled, i.e. to be sealed but not latched. Pressure testing may not reveal that latching of the components has not taken place, and the two components may separate and leak at a later stage in the life of an assembly. This is a particular problem in respect of fluid lines on motor vehicles which are subjected to vibration during service. In this coupling, on disassembly the seal will remain in position on the tube and the outer circumference will be unrestrained. Because such seals tend to swell during use, reassembly can then cause a problem.